1. Field of the Invention
This invention relates to a power conversion system wherein AC power of a single-phase AC power source is converted into DC power by a power converter. It more particularly relates to a power conversion system constructed so that compensatory control is performed in order that the power fluctuation of the single-phase AC power source is absorbed on the DC side.
2. Description of the Related Art
FIG. 19 is a layout diagram showing an example of a power conversion system conventionally employed for electric vehicles etc. which receive AC power from AC feeder line. In the drawing, SUP is a single-phase AC power source (single-phase power source), LS is an AC inductor, CNV is a pulse width modulation control converter (PWM converter) that converts the AC power to DC power, Cdo is a DC smoothing capacitor, INV is a pulse width modulation control inverter (PWM inverter) that converts the DC power to AC power, and IM is a three-phase induction motor.
The control circuit is constituted of: a DC voltage detector ISO, an input current detector CTS, an AC voltage detector PTS, motor current detectors CTU, CTV and CTW, a DC voltage control circuit AVR, an input current control circuit ACRS, a pulse width modulation control circuit PWMC1 for PWM converter CNV, a speed detector PG, a speed control circuit SPC, a load current control circuit ACRL, and a pulse width modulation control circuit PWMC2 for PWM inverter INV.
PWM converter CNV controls an input current Is that is supplied from single-phase AC power source SUP such that a voltage Vd that is applied to DC smoothing capacitor Cdo is practically constant. At this point, power conversion can be achieved with little higher harmonics and with an input power factor of 1, by controlling input current Is to a sine wave of the same phase as a power source voltage Vs.
Using DC smoothing capacitor Cdo as DC voltage source, PWM inverter INV converts the DC voltage into a three-phase AC power of variable voltage and variable frequency, and drives induction motor IM by the three-phase AC power. The 300-class electric train "Nozomi" used on the Tokaido Shinkansen Railway is an example of such a system, in which AC power is fed from single-phase AC power source SUP. The AC power is converted into DC power, and the DC power is again converted into AC power for driving induction motor IM.
PWM converter CNV has been described in Transactions of the Institute of Electrical Engineers of Japan, vol. 107-D No. 3, 1987, "Consideration on High Quality AC Traction Motor Drives using PWM converters". PWM inverter INV is well known to those skilled in the art, so that, the detailed description of PWM converter CNV and PWM inverter INV may be omitted.
The prior art power conversion system described is subject to the following problems.
Specifically, when AC power is fed from single-phase AC power source SUP, the AC power fluctuates with a frequency which is twice that of the power source frequency. As a result, voltage Vd that is applied to DC smoothing capacitor Cdo also fluctuates with a frequency that is twice that of the power source frequency. The magnitude of this voltage fluctuation is proportional to the magnitude of the active power that is output by PWM inverter INV, and inversely proportional to the capacitance of DC smoothing capacitor Cdo.
Consequently, if the capacitance of DC smoothing capacitor Cdo is made larger, the fluctuation of DC voltage Vd obtained by conversion performed by single-phase PWM converter CNV can be reduced. However, this increases the weight and size of the power conversion system, and increases its cost. In particular, in the drive system of an electric vehicle, the weight and size of the power conversion system should desirably be as small as possible. It was therefore necessary to allow a certain degree of DC voltage fluctuation.
However, this fluctuation of DC voltage Vd affects the inverter side, producing fluctuation of the output current of PWM inverter INV. For example, if the frequency of single-phase AC power source SUP is 50 Hz, the fluctuation frequency of DC voltage Vd is 100 Hz. In this case, as the output frequency of PWM inverter INV approaches 100 Hz, the fluctuation of the output voltage becomes large, giving rise to the beat in the current of motor IM. This results in pulsation of the torque generated by motor IM, causing vibration and noise.
Also, the maximum value of the voltage that is applied to the semiconductor devices constituting PWM converter CNV and PWM inverter INV is increased by the amount of the fluctuation of DC voltage Vd, making it necessary to employ semiconductor devices capable of withstanding higher voltages. This increases the weight and size of the device, thereby increasing its cost.